sophisticated 2-D and 3-D models for surge, waves, and other coastal phenomena (e.g., Sheng and Alymov, 2002; IPET, 2008). Whether any of these new models would improve FEMA’s modeling process is just beginning to be assessed in test beds. For example, the test bed led by the University of Florida is comparing four research storm surge and inundation models as well as the flood maps produced using the models.2 A comprehensive uncertainty study could help identify opportunities to increase the accuracy of coastal flood studies and priorities for improving FEMA’s coastal flood modeling and mapping methods.
Recommendation. FEMA should commission an external advisory group to conduct an independent, comprehensive assessment of coastal flood models to identify ways to reduce uncertainties in the models and to improve the accuracy of BFEs.
Such an assessment could consider factors such as
Performance metrics and standards for storm surge models and wind fields,
The necessary size of the coastal domain for storm surge simulation,
The effectiveness of patches applied to the WHAFIS/CHAMP model, and
The level of uncertainty associated with current 2-D and 3-D models, probabilistic methods, and WHAFIS.
Coastal flood studies rely on models of atmospheric and ocean phenomena that originate far from shore and that change in the nearshore and onshore environment. Considerable progress has been made in modeling these phenomena and mapping coastal flood hazard over the last 30 years. The modeling changes were usually incorporated in the form of patches. Modeling methodology is now poised for a major step forward, enabled by the availability of more advanced models and increased computing power, and sped by the need to better understand and represent coastal flood processes in the wake of Hurricane Katrina.
The key to improving coastal flood maps lies in improving the coastal flood models that are used to calculate the BFE, improving estimates of hurricane return period, and gathering more accurate pre- and post-storm topographic data. Published studies comparing WHAFIS with 2-D surge and wave models suggest that coupled 2-D surge and wave models yield more accurate BFEs, and the committee endorses their use. Other models emerging from the research community offer new or enhanced capabilities—such as those for calculating the effect of waves on storm surge and the effect of levees, marshes, or dunes on storm surge and waves—but they have not been compared to one another or to FEMA models to determine whether incorporating them into mapping practice would significantly improve the accuracy of coastal flood maps. A comprehensive model intercomparison study would help focus effort on which models should be further developed and adopted into FEMA methodology. The ultimate goal would be to use coupled models of storm surge, waves, and the effects of surface roughness, erosion, and overtopping or failure of topographic features to calculate the 1 percent annual chance stillwater elevation, wave setup, wave crest, and base flood elevation simultaneously. Similarly, cost comparisons of recent coastal mapping studies in Louisiana, Mississippi, and North Carolina—which were not available at the time of writing of this report—with older studies would help FEMA choose which new models are most cost-effective to pursue.